The present invention relates to testing, and more specifically, to optical probing of integrated circuits.
Many advancements have been made in the use of optical probing techniques used for testing integrated circuits; see, e.g., Paniccia, U.S. Pat. No. 5,895,972, Apr. 20, 1999 (xe2x80x9cPanicciaxe2x80x9d), incorporated by reference herein in its entirety. As described in Paniccia, flip-chips (integrated circuits which are mounted face down in the package) can be optically tested using laser probing. This is because the backside surface of the die of the integrated circuit (xe2x80x9cICxe2x80x9d) device under test (xe2x80x9cDUTxe2x80x9d), generally having a silicon substrate, is accessible for the purposes of laser or optical probing and/or for detecting photon emissions emitted from an active DUT. (This of course requires that the package be opened up.) Thus optical probing involves detecting optically electrical activity in the DUT while it is operating.
Although such optical probing is not limited to flip-chips, the backside of the die of the DUT must be visible to the optical probe so that waveform measurements can be made. During the testing or debugging of a DUT, it is generally desirable to operate the DUT at its full operating capacity and speed. Since the electric power consumption in, e.g., a microprocessor, is typically high, it is known in the art to exhaust heat created by the operation of the DUT to maintain an acceptable operating temperature; otherwise the DUT may be damaged.
FIG. 1A illustrates in a side view the approach of Paniccia (taken from his FIG. 5A) to dissipating heat from a flip-chip mounted integrated circuit die DUT 102. DUT 102 is conventionally mounted, via a plurality of ball bonds designated collectively as 107, to its package 110 having pins designated collectively as 115 used for coupling signals to/from and power to DUT 102. Pins 115 are electronically connected to contacts on a conventional test head 101 that also mechanically supports DUT 102 and the associated heat dissipating structures. Test head 101 also supplies the signals to and receives signals from the pins 115 and supplies power thereto. An infrared transmissive heat conductor film 120 is disposed over the backside surface 105 of DUT 102. A heat sink 150 (i.e., a structure capable of absorbing dissipating heat, being typically of large thermal mass and made of, e.g., copper) is in thermal contact with the outer edges of the heat conductor""s top surface. Heat conductor film 120 is of a material that is transmissive of infrared wavelength light, e.g., diamond. (The remainder of the test apparatus is not shown here.) FIG. 1B illustrates a plan view of the FIG. 1A apparatus.
Although U.S. Pat. No. 5,895,972 describes the removal of heat from an active integrated circuit, we have recognized the additional advantage of actively controlling temperature of the DUT during optical testing. In other words, if during optical probing or other optical testing of the active DUT the electric power consumption and hence heat output of the DUT fluctuates, we have recognized there will be a corresponding fluctuation in the temperature of the DUT. This fluctuation in temperature will adversely affect the test results obtained by interfering with the propagation of the probing beam and/or the detected light. Passive observation of photons emitted from transistors that are switching is also known.
This fluctuation in temperature will also adversely affect the test results due to the fluctuating operating conditions of the IC die. The goal is to collect the data from the die while maintaining the die at a specific temperature, with only small temperature variations.
Therefore, recognizing that the DUT temperature should be kept constant, we determined there is needed a method and an apparatus for controlling the temperature of an integrated circuit DUT during its active operation while simultaneously allowing the DUT to be probed, or otherwise tested in accordance with an optical-based testing technique.
Therefore, we describe here an apparatus and method for optical based testing through or at the backside surface of an active IC semiconductor substrate (die) that permits active temperature control of the DUT.
In one embodiment, the present apparatus is used with a an integrated circuit DUT having an optically transmissive heat conductor in thermal contact with, e.g., the backside surface of the DUT. The optically transmissive heat conductor is in thermal contact with an additional heat conductor structure, which in turn is in thermal contact with a thermoelectric device. The thermoelectric device, capable of heating or cooling, advantageously controls the temperature of the active IC DUT during optical probing and/or testing.